I know that other nations, like Germany and Japan changed caliburs late in the war, but why was America the only one favoring the high calibur machine gun rounds from the get go? As for the rebuttle that Germany and Britain used 20mm, that is not a machine gun. That is a cannon.

The americans had some trouble with 20mm weapons, so most aircraft lacked them, i think the first to carry any were maybe the P-38. They liked the .50, and had weapons in the army and navy that also used the same calibre, maybe to ease production and logistics and distribution problems, by focusing on one calibre. Many early american fighters had .30 cal guns though too. The P-40 initially did, or the AVG had some that did i know that, the P-39 did, and many bombers were armed defensively with them.

The USAAF found itself in a postion where the .50 calibre was really the only feasabile weapon it could arm it planes with. There are a couple of reasons;

US attempts to mass produce a 20mm cannon were generally unsucessful. There was an attempt to alter the gun to be produced to US specifications which never really worked. Most of the 20mms were shipped to the UK. The British found them unsatisfactory compared to their own Hispano production. They had a much higher rate of jamming and breakdowns than UK produced cannon. See this article http://www.quarry.nildram.co.uk/US404.htm for a good history of US attempts to produce the Hispano.

Similarly, the attempt to produce an indigenous Very Heavy Machine Gun also failed. There was a program to re-engineer the Mg151/15 into a .60 calibre (15.3 mm), very high velocity machine gun. Like the 20mm program, it really didn't result in a workable design isn the wartime period.

The other reason that the USA stuck with the .50 is basic economics. The .50 was an aqequate, if not excellent, air to air weapon. It was already standard fit on most US fighters. Upon entering the war there were significant stocks of weapons and ammunition available for increased production.

So, why change to a new weapon, when the .50 cal was 'good enough' for the job at hand? Switching to a new weapon would poentially disrupt production, require redesigns of weapons bays and render stockpile of weapons unnneeded.

More than anything the 'sunk' costs of the .50 kept it as the standard weapon for WW2 for the USA. My personal feeling was that the .50 was kept for reasons of structural inertia. There was
a) singinficant infrastructure developed before the war to produce the weapon (production facilities, usage, logistics),
b) existing designs incorporated the weapon as standard
c) the future of other weapons programs were uncertain in their results
d) logistics concerns of new ammunition types

You also can't forget that the VVS (Russian Airforce) used the UB and UBS 12.7mm weapons as standard from the beginning. They were fitted to the I-153, I-16 and LaGG-3 at the start of Operation Barbarossa. It was also fitted to the Yak family of fighters. The UB and UBS were the best heavy machine-guns of the war ( I tend to count the MG 151/15 as a cannon). They fired a longer, heavier shell than the Browning, with more than twice the HE capacity, at a faster rate of fire, with similar muzzle velocity, from a lighter gun. Pound for pound the UB was about 1/3rd more effective than the .50 calibre Browning.

Simple, we started replacing the 12.7 mm (50 cal) Browning when it was found that the speeds of aircraft were getting to the point where only a short burst could be expected to hit. Once taht happened, weight of fire per second was more important than rate of fire per second.

By the time we mounted 30 mm cannons, one hit in a vital area would disable a plane, if not shoot it down.

The A-10 shoots depleted Uranium rounds that have enough muzzle energy to penetrate a tank with one round.

Think of the Me 262. It was 100 mph faster than the P-51. In a dogfight, if there ever WAS one (which I doubt), the P-51 could not hit the Me 262 with enough rounds in a short burst to do significant damage before the plane was out of the line of sight. It took hits on the engines to slow the Messerschmitt down.
As long as you have only a small chance of a hit, a more energetic round is indictaed.

Think of the Me 262. It was 100 mph faster than the P-51. In a dogfight, if there ever WAS one (which I doubt), the P-51 could not hit the Me 262 with enough rounds in a short burst to do significant damage before the plane was out of the line of sight. It took hits on the engines to slow the Messerschmitt down.

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WTF are u talking about dude??? The -262 didnt fly around with his engines at full throttle...

I think its more the only examply he could come up with that would relate to the topic at hand, from world war two anyway. I would have used the Corsair and the Mig-15 as my example, but thats me.
But when you look at whats said it does make sense, you could throw a bunch of 12.7mm rounds at a Mig-15, and the damn thing would just keep on flying (damn russians and there rugged construction) but the Mig, armed with a 37mm cannon and two 23mm cannon would rip a P-80 or F-86 to shreds with a small amount of hits. They were built to intercept bombers, and were therefore armed with good quality russian cannon for the job, but admittedly with much less ammunition than the six 12.7mm of the F-86.

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Simple, we started replacing the 12.7 mm (50 cal) Browning when it was found that the speeds of aircraft were getting to the point where only a short burst could be expected to hit. Once taht happened, weight of fire per second was more important than rate of fire per second.

By the time we mounted 30 mm cannons, one hit in a vital area would disable a plane, if not shoot it down.

The A-10 shoots depleted Uranium rounds that have enough muzzle energy to penetrate a tank with one round.

Think of the Me 262. It was 100 mph faster than the P-51. In a dogfight, if there ever WAS one (which I doubt), the P-51 could not hit the Me 262 with enough rounds in a short burst to do significant damage before the plane was out of the line of sight. It took hits on the engines to slow the Messerschmitt down.
As long as you have only a small chance of a hit, a more energetic round is indictaed.

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Um we are talking about WW2 here and at the end of the War the .50 Cal was still the standard armament on US aircraft. It was effective still at that time.

As for the A-10 and depleted Uranium Rounds that is completely different technology and is irrelevent to WW2 aviation.

As for the Me-262, single shots of just about anything in the engine would kill the aircraft not just slow it down. The engines were the Me-262's real weak point. The engines were not reliable and failed often eneogh on there own.

A still believe that if the USA had been forced to take on a plane such as the B17 or B29 the 0.5 would have been found badly lacking. All forces that faced these planes went up to 30mm guns and the 0.5 would ave lacked the punch.
The 0.5 was only effective because of the planes it had to fact i.e. single engined planes, mainly pre war twins, and highly inflammable Jap bombers. Even B25's and B26's wold have proved a handful.

One point also, the British were indeed interested in arming-up with .50's, about the time after the Lancaster came on stream, but by then the US was into the War, and their demand delayed a serious introduction earlier...The British .303's had a high rate of fire, a ''good spray'', but they eventually had .50's in Spitfires [with cannon] and Lancasters got two of them in their rear-turrets later on too....The earlier variants of Mustangs that the RAF got were eventually fully up-graded from .30 cal to .50's too......